1. Field of the Invention
The present invention relates to compositions and methods for inhibiting the precipitation and deposition of scale-forming salts in an aqueous system.
Most industrial and commercial water contains alkaline earth metal cations, such as calcium, magnesium, etc., and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. These precipitates are alkaline earth metal scales. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction product, a solid phase of calcium carbonate scale will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on heat transfer surfaces in contact with aqueous streams, they form scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal. Alkaline earth metal scales commonly form on the metallic surfaces of apparatuses used for thermal treatment of aqueous solutions and suspensions. By alkaline earth metal scales is meant scales including but not limited to calcium carbonate, magnesium carbonate, calcium phosphate, and calcium sulfate. These scales form frequently in the tubes of heat exchangers and on other heat exchange surfaces.
Heretofore, alkaline earth metal scale inhibition has been facilitated by the use of anionic polyelectrolytes such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonated styrenes. See, for example, U.S. Pat. Nos. 4,640,793; 4,650,591; and 4,671,888. However, when used as threshold alkaline earth metal scale inhibitors, large dosages of these polymers are required, which in turn increases operating costs.
Scale-forming compounds can also be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Generally, this requires many times as much chelating or sequestering agent as cation, since chelation is a stoichiometric reaction, and these amounts are not always desirable or economical. However, almost 50 years ago, it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating.
When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale-forming cation (stoichiometric), it is said to be present in "threshold" amounts. See, for example, Hatch and Rice, Indust. Eng. Chem., 31, 51-53 (1939); Reitemeier and Buehrer, J. Phys. Chem., 44 (5), 535-536 (1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305.
Generally, sequestering takes place at a weight ratio of threshold active compounds to scale-forming cation components of greater than about 10:1, depending on the anion components in the water. Threshold inhibition generally takes place at a weight ratio of threshold active compound to scale-forming cation components of less than about 0.5:1.0.
More recently, attention has been focused on controlling scaling under severe conditions, where conventional treatments such as those described above do not provide complete scale control. Current technology in scale control can be used to inhibit CaCO.sub.3 scale up to 100 to 120 times calcite saturation, i.e., a water containing Ca.sup.2+ and CO.sub.3.sup.2- present at 100 times their solubility limit, can be prevented from precipitating as calcium carbonate scale using substoichiometric amounts of an inhibitor.
Severity of the scaling tendency of a water sample is measured using the saturation index, which may be derived in accordance with the following equation: ##EQU1## where SI is the saturation index for calcium carbonate, (Ca.sup.2+) is the concentration of free calcium ions, (CO.sub.3.sup.2-) is the concentration of free carbonate ions, and .sup.K spCaCO.sub.3 is the solubility product constant for CaCO.sub.3. All of the quantities on the right side of the above equation are adjusted for pH, temperature and ionic strength.
It has been discovered that, surprisingly, when the scale inhibition compositions of the present invention are employed, in accordance with which several types of functional groups are combined into a single molecule, that scale control under these severe conditions is attainable.
One of the particular advantages of the scale inhibiting compositions of the present invention is the exceptional calcium tolerances which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions (Ca.sup.2+). One of the parameters of scale control under severe conditions is pH. As pH increases, calcium tolerance decreases rapidly for traditional CaCO.sub.3 threshold inhibitors, e.g., HEDP and AMP. These inhibitors precipitate with calcium at alkaline pH's, rendering them useless as threshold scale inhibitors. The addition of sulfonate groups in accordance with the present invention was intended to increase calcium tolerance. It was considered that if the sulfonated phosphonate would be soluble in the presence of Ca.sup.2+ at high pH, it would be available to inhibit CaCO.sub.3 scale. The addition of the sulfonate group has, in fact, increased the phosphonate's solubility with calcium at pH 10.2, as shown in the data further below. Performance has also been improved.
2. Brief Description of the Prior Art
The chemical compounds used in the scale inhibiting compositions of the present invention are known compounds. However, there is no teaching in the art of which applicants are aware that would in any way suggest their use in corrosion and scale inhibiting compositions in accordance with the present invention.
Sommer et al. U.S. Pat. Nos. 4,216,163 and 4,250,107 disclose N-sulfo alkane amino alkane phosphonic acids which are said to be sequestering agents useful, e.g., in decreasing the hardness of aqueous systems. While these compounds combine lower alkylene phosphonic acid groups and carboxy lower alkylene groups or alkane sulfonic acid groups in a single molecule, there is no suggestion of the particular compounds of the present invention or of their surprising effectiveness in inhibiting scale under severe conditions.